Extend example.

coupler/opc-ua-server/ic2_set_relays_example.c

-// Example controling MOD-IO's relays' state 
-// Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface
+/*
+  Example controling MOD-IO's relays' state 
+  Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface
+  
+  To compile:
+    root@olimex-rt:~/osie/coupler/opc-ua-server# gcc ic2_set_relays_example.c -o ic2_set_relays_example
+    root@olimex-rt:~/osie/coupler/opc-ua-server# ./ic2_set_relays_example
+*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -12,6 +19,7 @@ int main()
 {
    int file;
    int adapter_nr = 1; /* probably dynamically determined */
+   int addr = 0x58; /* The I2C slave's address */
    char filename[20];
 
    // step 1: open device
@@ -24,7 +32,6 @@ int main()
    }
   
    // step 2: address the slave by its address 
-   int addr = 0x58; /* The I2C address */
    if (ioctl(file, I2C_SLAVE, addr) < 0) {
     /* ERROR HANDLING; you can check errno to see what went wrong */
     printf("Error addressing i2c slave.");
@@ -36,7 +43,8 @@ int main()
    __s32 res;
    char buf[10];
    buf[0] = reg;
-   buf[1] = 0x05; //0x00 -all off, 0x0F - all 4 on
+   buf[1] = 0x07; // the most right bits represent 4 relays' state: for example 00001111 - means all ON
+                  // send as a hexadecimal number
    buf[2] = 0x65; // seems irrelevant the value
    if (write(file, buf, 3) != 3) {
      /* ERROR HANDLING: i2c transaction failed */